It has been known for many years that a color image can be produced by means of reaction involving contact between an electron-donating or proton-accepting colorless organic compound (hereinafter referred to as a color former) and an electron-accepting or protonreleasing solid acid (hereinafter referred to as a color developer). This phenomenon is embodied in pressure-sensitive copying paper as described in U.S. Pat. Nos. 2,505,470, 2,505,489, 2,550,471, 2,548,366, 2,712,507, 2,730,456, 2,730,457, 3,418,250, and 3,672,935. A printing method has also been proposed that prepares a sheet coated with color developer and produces a color image on that sheet by supplying an ink containing a color former; this technique is disclosed in West German Patent Application (OLS) No. 1,939,962.
The color developer has the properties defined above and is selected from among clays, phenolic resins, and metal salts of aromatic carboxylic acids. Since these color developers are usually coated in a uniform thickness on the entire surface of a support, the non-image areas of the sheet of color developer are desensitized by printing or otherwise coating a composition containing an appropriate desensitizer.
Detailed descriptions of such desensitizers are set forth in U.S. Pat. Nos. 2,777,780, 3,890,156, 3,931,430, 3,952,117, 4,012,538, 4,022,624, and 4,101,690; West German Patent 2,526,592; West German Patent Application (OLS) Nos. 2,359,079 and 2,727,194; Belgian Patent No. 804,221; Japanese Patent Publication Nos. 29546/71, 23850/74, 14571/75, and 29365/75; and Japanese Patent Application (OPI) Nos. 125018/77 and 67291/81 (the term "OPI" as used herein means a "published unexamined Japanese Patent Application"). Specific examples of the desensitizer include dodecyl trimethylammonium chloride, dodecylamine, 2,4,4-trimethyl-2-oxazoline, xylenediamine, polyoxyethylene alkylamine, polyoxyethylene alkylether, polyoxyethylene alkylphenyl ether, polyethylene glycol, polypropylene glycol, glycidyl ether adducts of amines, etc.
These desensitizers, however, are not completely satisfactory in their desensitizing effects and their effectiveness is particularly low with respect to fluoran-based color formers such as 3,6-bis-diphenylaminofluoran and 3-diethylamino-7-dibenzylaminofluoran. If a color former is brought into contact with the sheet of color developer after it is coated with a desensitizer in the non-image areas, those areas initially seem to be completely desensitized, but if the sheet is exposed to light (particularly sunlight), it often occurs that a color image appears on the non-image areas. In order to avoid this problem, a very thick coating of the desensitizer must be formed on the sheet of color developer by printing, but then the printed surface dries so slowly that the printing speed cannot be increased to an industrially acceptable level.
In addition, if characters are written or printed with a color ink on the surface of the sheet of color developer that has been coated with an increased amount of desensitizer, the resulting ink image will tend to undergo extensive discoloration or may be blurred.